1. Field of the Invention
The present invention relates to use of the Global Positioning System (GPS); and, in particular, to techniques for circumventing jamming of GPS receivers.
2. Description of the Related Art
The Global Positioning System (GPS) is widely known and used to provide precision positioning information anywhere on Earth. GPS includes a constellation of earth orbiting satellites that transmit coded signals, and a portable unit called a GPS receiver to receive those coded signals. The GPS receiver includes a radio frequency receiver, a processor, a clock, and data used to determine a position of the portable unit based on signals received from four or more of the GPS satellites. The GPS receiver measures raw ranges to the four or more satellites and solves for a position and clock offset that fits the measured ranges. Because the signals emitted by the satellites are digital, the system works even at low power. The low power is an advantage for conserving power on the satellites that must transmit over tens of thousands of miles from an orbit about 12,000 miles above the surface of the Earth. Positions can be determined within a few tens of meters both horizontally and vertically.
The high precision position information provided by GPS is valuable in a number of commercial and military scenarios. Opponents hostile to an enterprise's commercial or military goals might decide to deny the benefits of GPS positions to the enterprise by interfering with the GPS transmissions. Such an opponent may attempt to jam the GPS receivers with spurious signals. The spurious signals might have large enough amplitude to bury the low power GPS signals and thus prevent the determination of range to one or more of the GPS satellites. The spurious signals might spoof the GPS receiver by including data that appears to be from GPS satellites but that gives one or more raw ranges that prevent a proper solution for position and clock offset. Without positioning information, the enterprise may be unable to achieve one or more goals.
Based on the foregoing description, there is a clear need for a system that would circumvent jamming of global positioning system receiver so that an enterprise's goals can still be achieved in the presence of such jamming.